STEVE ALLEN / SCIENCE PHOTO LIBRARY STEVE ALLEN / SCIENCE PHOTO LIBRARY
Perito Moreno Glacier. This glacier is in Patagonia, Argentina. Periodically, it moves out across a lake (seen at lower right), forming a large dam behind which the level of the lake rises. When the pressure builds up to high enough levels, the dam ruptures in spectacular fashion. This rupture event occurs every 2-5 years.
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